The invention relates generally to internal cooling of rotating turbine blades (hereinafter xe2x80x9crotor bladesxe2x80x9d) and, more particularly, to turbulator configurations for inner surfaces of cooling passages within rotor blades.
In gas turbine engines, hot gases from a combustor are used to drive a turbine. The gases are directed across rotor blades, which are radially connected to a rotating turbine rotor disk. Such gases are relatively hot. The capacity of the engine is limited to a large extent by the ability of the rotor blade material to withstand the resulting temperature and stress. In order to decrease blade temperature, thereby improving thermal capability, it is known to supply cooling air to hollow cavities within the blades. Typically one or more cooling passages are formed within a blade with a coolant (such as compressor discharge air) supplied through an opening at the root of the blade and allowed to exit through cooling holes strategically located on the blade surface and/or blade tip. The cooling passages provide convective cooling inside the blade and film-type cooling on the surface of the blade. Many different cavity geometries have been employed to improve heat transfer to the cooling air inside the blade. For example, cooling passages typically have circular, rectangular, square or oblong transverse cross-sectional shapes.
One known rotor blade cooling circuit includes a plurality of unconnected longitudinally-oriented passages (hereinafter xe2x80x9cradial cooling passagesxe2x80x9d) extending through an airfoil of the rotor blade. Each radial cooling passage receives cooling air from near the root of the airfoil and channels the air longitudinally toward the tip of the airfoil. Other cooling circuits are serpentine, comprising a plurality of longitudinally-oriented passages which are series-connected to produce serpentine flow. For either cooling circuit, some air exits the blade through film cooling holes near the blade""s leading edge and some air exits the blade through trailing edge cooling holes.
It is known that for a rotor blade, the flow of coolant inside the cooling passage includes a secondary flow pattern caused by Coriolis (rotation) forces. More precisely, for a rotor blade, the Coriolis force increases the heat transfer along certain walls of the passage and decrease the heat transfer along other walls of the passage as compared with a stationary airfoil. Briefly, the Coriolis force is proportional to the vector cross product of the velocity vector of the coolant flowing through the cooling passage and the angular velocity vector of the rotor blade. Accordingly, the Coriolis force compresses the coolant against one side of the passage increasing the heat transfer at that side while decreasing the heat transfer at the opposite side. This creates an uneven transverse cross-section blade temperature profile, which creates hot areas that must be compensated for by, for example, increasing the cooling flow. Increasing the cooling flow could be accomplished by bleeding off more engine compressor air, but this would reduce the engine""s efficiency by reducing the number of miles flown for each gallon of fuel consumed.
It is further known to provide turbulence promoters or xe2x80x9cturbulatorsxe2x80x9d in the cooling passages of rotor blades to generate turbulence near the cooling passage wall. By creating turbulence in the vicinity of the turbulator, heat transfer between the coolant and the cooling passage wall is enhanced.
Currently, radial cooling passages are formed in large turbine blades using shaped-tube electrochemical machining (STEM), in which the blade functions as an anode, while a plurality of drilling tubes function as cathodes in the STEM process. Briefly, the blade is flooded with an electrolyte solution from the drilling tubes, and material is deplated from the blade in the vicinity of the leading edge of the drilling tubes to form the cooling passages. The STEM process is modified to form turbulated ridges in the cooling passages. One common modified STEM method is termed xe2x80x9ccyclic dwelling.xe2x80x9d With this technique, the drilling tube is first fed forward, and then the advance is slowed or stopped in a cyclic manner. The dwelling of the tool that occurs when the feed rate is decreased or stopped creates a local enlargement of the passage diameter, or a bulb. The cyclic dwelling, for which cyclical voltage changes may be required, causes ridges to be formed between axially spaced bulbs. These ridges are the turbulators.
In addition to being inefficient (for example, the dwell time to form a bulb can exceed the time to drill a straight-walled cooling passage), the cyclic dwelling method produces turbulators that extend circumferentially around the cooling passage wall (hereinafter xe2x80x9cannular turbulators.xe2x80x9d) The annular turbulators are deficient in that they do not exploit the secondary flow caused by the Coriolis force.
Accordingly, there is a need in the art for new and improved turbulator configurations for radial cooling passages in rotor blades. New and improved turbulator configurations that cooperate with the secondary flow caused by the Coriolis force would enhance heat transfer from the cooling passage wall to the coolant, thereby facilitating reduced cooling flow (i.e., bleeding off less compressor air), which in turn increases turbine engine efficiency. There is a corresponding need for rotor blades having radial cooling passages that incorporate the improved turbulator configurations. Such rotor blades would advantageously have higher heat transfer coefficients, enhancing turbine engine efficiency. Moreover, there is a corresponding need for a method and tool to efficiently form the improved turbulator configurations.
Briefly, in accordance with a turbulator configuration embodiment of the present invention, a curved turbulator configuration is provided in a radial cooling passage of an airfoil, the radial cooling passage defined by at least a leading wall and a trailing wall, the airfoil including a tip and a root. The curved turbulator configuration includes a number of spaced curved turbulator pairs positioned along a center-line on an inner surface of the leading wall, and a number of spaced complementary curved turbulator pairs positioned along a center-line on an inner surface of the trailing wall.
An airfoil embodiment of the invention includes a tip including at least one exit hole. The airfoil further includes a root and a body extending between the tip and the root. The body includes a pressure side and a suction side and a leading wall on the suction side and a trailing wall on the pressure side. The airfoil further includes at least one radial cooling passage extending through the body between the tip and the root. The radial cooling passage is defined by at least an inner surface of the leading wall and an inner surface of the trailing wall. The airfoil further includes the curved turbulator configuration, according to the turbulator configuration embodiment, integrated with the inner surfaces of the leading and trailing walls. The exit hole is connected to the radial cooling passage and is configured to vent coolant from the airfoil after the coolant flows through the radial cooling passage.
A rotor blade embodiment of the invention includes a shank and the airfoil according to the airfoil embodiment. The airfoil is attached to the shank.
An electrochemical machining method embodiment of the invention is provided for forming the curved turbulator configuration on the inner surface of the leading wall of the airfoil and on the inner surface of the trailing wall of the airfoil, the inner surfaces defining a radial cooling passage extending between the tip and the root of the airfoil. The electrochemical machining method includes positioning an electrode in a predrilled hole in the airfoil, the electrode comprising a conductive core and an insulating coating, the electrode having a leading face and a trailing face. The insulating coating provides a curved turbulator pattern on the leading face and a complementary curved turbulator pattern on the trailing face. The method further includes machining at least one pair of curved turbulators on the inner surface of the leading wall using the curved turbulator pattern and at least one pair of complementary curved turbulators on the inner surface of the trailing wall using the complementary curved turbulator pattern. The pairs of curved turbulators and complementary curved turbulators are machined simultaneously by passing an electric current between the electrode and the airfoil while circulating an electrolyte solution through the predrilled hole.
An electrode embodiment of the invention for forming the curved turbulator configuration in the radial cooling passage within the airfoil includes a leading face having a curved turbulator pattern, a trailing face having a complementary curved turbulator pattern, a conductive core, and an insulating coating disposed on the conductive core. The insulating coating is partly removed. The curved turbulator pattern includes a number of pairs of curved exposed portions of the conductive core, and the complementary curved turbulator pattern includes a number of pairs of complementary curved exposed portions of the conductive core.
Another electrode embodiment of the invention for forming the curved turbulator configuration in the radial cooling passage within the airfoil includes the leading face having a curved turbulator pattern, the trailing face having a complementary curved turbulator pattern, the conductive core, and the insulating coating disposed on the conductive core. The insulating coating is partly removed to include a number of pairs of curved portions of the insulating coating on the leading face and a number of pairs of complementary curved portions of the insulating coating on the trailing face. Each of the curved portions and the complementary curved portions is surrounded by an exposed region of the conductive core. For this embodiment, the curved turbulator pattern comprises the pairs of curved portions of the insulating coating, and the complementary curved turbulator pattern comprises the pairs of complementary curved portions of the insulating coating.